Of the many hydroconversion processes known in the art, hydrocracking is becoming increasingly important since it offers product flexibility together with product quality. As it is also possible to subject rather heavy feedstocks to hydrocracking it will be clear that much attention has been devoted to the development of hydrocracking catalysts.
Modern hydrocracking catalysts are generally based on zeolitic materials which may have been adapted by techniques like ammonium ion exchange and various forms of calcination in order to improve the performance of the hydrocracking catalysts based on such zeolites.
One of the zeolites which is considered to be a good starting material for the manufacture of hydrocracking catalysts is the well-known synthetic zeolite Y as described in U.S. Pat. No. 3,130,007 issued April 21, 1964. A number of modifications has been reported for this material which include, inter alia, ultrastable Y (U.S. Pat. No. 3,536,605 issued Oct. 27, 1970) and ultrahydrophobic Y (U.K. Patent Application GB-A-2,014,970, published Sept. 5, 1979). In general, it can be said that the modifications cause a reduction in the unit cell size depending on the treatment carried out.
The ultrahydrophobic Y zeolites as described in GB-A-2,014,970 are also referred to in European Patent Application EP-B-28,938 published May 20, 1981, and European Patent Specification EP-B-70,824 published February 5, 1986, as suitable components for hydrocracking catalysts. From said publications it appears that such zeolites have an intrinsically low water adsorption capacity. Water adsorption capacities below 5% (EP-B-28,938), respectively 8% (Specification EP-B-70,824) by weight of zeolite are considered to be the maximum levels acceptable and it is confirmed experimentally in EP-B-28,938 that a water adsorption capacity of 8.5% by weight on zeolite causes a drastic decrease in selectivity.
In European Patent Application EP-A-162,733 published Nov. 11, 1985, zeolite Y components for hydrocracking catalysts are described which must possess a rather stringent pore diameter distribution which in essence means that at least 80% of the total pore volume is made up of pores having a diameter of less than 2 nm, and preferably at least 85% of the total pore volume is made up of pores having a diameter of less than 2 nm.
In U.K. Patent Application GB-A-2,114,594 published Aug. 24, 1983, a process for the production of middle distillates is disclosed wherein use is made of catalysts comprising so-called expanded pore faujasitic zeolites. The pore expansion referred to in said patent specification has been obtained by firstly steaming the faujasitic zeolite at a temperature of at least 538.degree. C., in particular at a temperature above 760.degree. C., followed by contacting the steamed faujasitic zeolite with an acid, preferably an acid having a pH less than 2. It should be noted that the degree of crystallinity retained in the expanded pore zeolite dramatically decreases at increasing amounts of acid used (see FIG. 3 of GB-A-2,114,594). Since the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio substantially increases linearly with the amounts of acid used (see FIG. 2) it appears that the crystallinity of the faujasitic zeolites treated according to the process described in GB-A-2,114,594 intrinsically decreases at increasing SiO.sub.2 /Al.sub.2 O.sub.3 molar ratios.
It has now been found that the presence of certain modified Y zeolites together with an amorphous cracking component in hydrocracking catalysts gives an unexpected high selectivity to the desired product(s) combined with a significantly lower gas make than experienced thus far with catalysts based on Y zeolite. Also substantial amounts of polynaphthenic compounds present in the feed to be processed, which compounds are notoriously difficult to process, if at all, can be conveniently converted in the process according to the present invention.
It has now been found that the presence of amorphous cracking components has a significant impact on the conversion of polynaphthenic compounds. The expression "polynaphthenic compounds" as used herein should be understood as relating to polynaphthenic compounds which as measured by mass spectroscopy have four or more rings in their respective structures which are predominantly condensed. Moreover, it was found that the quality of the product(s) was improved despite a lower hydrogen consumption. These improvements are even more remarkable since they can be achieved with catalysts showing a higher activity than thus far achievable with Y type zeolites.